Kasegalik Stromatolites Belcher Islands, Nunavut - Canada

A stromatolite is a layered rock structure built by communities of microbes growing as mats in shallow water. As the mats trapped sediment and grew upward toward sunlight, they left a layered record of their growth. Stromatolites are among the earliest evidence of life on Earth.

The Strelley Pool and North Pole stromatolites of Western Australia are older, about 3.5 to 3.4 billion years old, and widely accepted as biological, but the microbes that built them are not preserved as identifiable cells. Kasegalik stromatolites, about 2.02 billion years old, preserve their builders: the cyanobacterium Eoentophysalis belcherensis, named to genus and species. The older stromatolites show that life was present; the Kasegalik shows who built them.

Each cell is only 3 to 5 micrometers across. As the cells divided, they wrapped themselves in nested gelatinous envelopes, forming small, tightly packed colonies. Their division pattern and colony structure are indistinguishable from the living cyanobacterium Entophysalis, which still builds microbial mats on tidal flats today, a striking example of biological stability across two billion years.

They also indicate that oxygen-producing cyanobacteria, the microbes credited with first adding oxygen to Earth's atmosphere, were building stromatolites by at least 2.02 billion years ago.

Early Earth's surface received far more ultraviolet radiation than today, so the earliest mat-building microbes likely needed protection, but the 3.5- to 3.4-billion-year-old Pilbara stromatolites do not preserve cells that could show it. Eoentophysalis belcherensis does: its colonies concentrated dark, sun-shielding pigment on their upward-facing envelopes, matching the pattern of scytonemin, the UV-screening pigment of living cyanobacteria.

Modern stromatolites in high-altitude Andean lakes, where ultraviolet exposure is extreme, rely on the same kind of protection, making them a living window into how these ancient microbes survived.

The enlarged images show the structure the microbes built, not the microbes themselves. The dark gray areas are small finger-like columns and rounded lobes of stromatolite growth, separated by paler chert and dolomite that filled the spaces between them. Fine red threads of iron oxide trace many column margins, and a pinkish-white mineral vein cuts straight across the structure, showing it formed long after the stromatolites grew.

Microfossils preserved in this specimen would be in the dark, fine-grained chert within the columns, where early silica replacement could capture the cells. Each cell is only 3 to 5 micrometers across.

Two thin layers of volcanic ash within the Kasegalik Formation contain zircon crystals that formed during the eruptions, and the steady decay of uranium to lead inside them acts as a natural clock. A 2019 study dated the ash layers at about 2.018 and 2.015 billion years, making these among the most precisely dated early microfossil-bearing rocks known.